fix: address PR review feedback on help texts and translations

- Replace 'speelronde' with 'spel' in index and season_add help
- Season settings help: remove incorrect claim name is editable, describe
  actual settings (Show Numbers, Confirm Answers)
- quiz_add help: rename 'XLSX-bestand' to 'Excel-bestand', add explanation
  of WAAR/ONWAAR (Dutch Excel) vs TRUE/FALSE (English Excel) for marking
  the correct answer
- quiz_answer_mapping help: remove em-dashes
- quiz_candidates help: clarify that multiple devices and mixed setups
  (multiple laptops, phones, or a mix) are supported
- quiz_question_bank_form help: change 'thema' to 'type vraag' for labels
- Rename 'Add from XLSX' to 'Import' in translation and template

translations/messages+intl-icu.nl.xliff

@@ -53,9 +53,9 @@
         <source>Add blank quiz</source>
         <target>Lege quiz toevoegen</target>
       </trans-unit>
-      <trans-unit id="2Qmgp4S" resname="Add from XLSX">
-        <source>Add from XLSX</source>
-        <target>Toevoegen vanuit XLSX</target>
+      <trans-unit id="2Qmgp4S" resname="Import">
+        <source>Import</source>
+        <target>Importeren</target>
       </trans-unit>
       <trans-unit id="MJD3m3Q" resname="Add label">
         <source>Add label</source>